This section is intended to provide a background to the various embodiments of the technology described in this disclosure. The description in this section may include concepts that could be pursued, but are not necessarily ones that have been previously conceived or pursued. Therefore, unless otherwise indicated herein, what is described in this section is not prior art to the description and/or claims of this disclosure and is not admitted to be prior art by the mere inclusion in this section.
Long Term Evolution (LTE) uses Orthogonal Frequency Division Multiplexing (OFDM) in the downlink and Discrete Fourier Transform (DFT)-spread OFDM in the uplink. The basic LTE downlink physical resource can thus be seen as a time-frequency grid as illustrated in FIG. 1a, where each resource element corresponds to one OFDM subcarrier during one OFDM symbol interval.
FIG. 1b illustrates a LTE time-domain structure. As shown in FIG. 2, in the time domain, LTE downlink transmissions are organized into radio frames of 10 ms. Each radio frame consists of ten equally-sized subframes of a length Tsubframe=1 ms.
Furthermore, the resource allocation in LTE is typically described in terms of resource blocks, where a resource block corresponds to one slot, e.g. 0.5 ms, in the time domain and 12 contiguous subcarriers in the frequency domain. A pair of two adjacent resource blocks in time direction, e.g. 1.0 ms, is known as a resource block pair. Resource blocks are numbered in the frequency domain, starting with 0 from one end of the system bandwidth.
The LTE Carrier Aggregation (CA) feature was introduced in Rel-10 and enhanced in Rel-11. It offers means to increase the peak data rates, system capacity and user experience by aggregating radio resources from multiple carriers that may reside in the same band or different bands and, for the case of inter-band Time Division Duplex (TDD) CA, may be configured with different UpLink (UL)/DownLink (DL) configurations. In Rel-12, CA between TDD and Frequency Division Duplex (FDD) serving cells is introduced to support UE connecting to them simultaneously. In Rel-13, Licensed-Assisted Access (LAA) has attracted a lot of interest in extending the LTE CA feature towards capturing the spectrum opportunities of unlicensed spectrum in the 5 GHz band. Wireless Local Area Network (WLAN) operating in the 5 GHz band nowadays already supports 80 MHz in the field and 160 MHz is to follow in Wave 2 deployment of IEEE 802.11ac. There are also other frequency bands, such as 3.5 GHz, where aggregation of more than one carrier on the same band is possible, in addition to the bands already widely in use for LTE. Enabling the utilization of at least similar bandwidths for LTE in combination with LAA as IEEE 802.11ac Wave 2 will support calls for extending the CA framework to support more than 5 carriers. The extension of the CA framework beyond 5 carriers was approved to be one work item for LTE Rel-13. The objective is to support up to 32 carriers in both UL and DL.
To support up to 32 carriers in DL, the Uplink Control Information (UCI) feedback, e.g. Hybrid Automatic Repeat Request (HARQ)-Acknowledgement (ACK) bits will increase significantly. For each DL subframe, one or two HARQ-ACK bits per carrier need to be reported depending on if spatial multiplexing is configured. Hence, for FDD, there can be up to 64 HARQ-ACK bits if there are 32 DL carriers. The number of HARQ-ACK bits for TDD is even larger, up to hundreds of bits depending on the TDD configuration. Therefore, new Physical Uplink Control CHannel (PUCCH) format(s) supporting larger payload is necessary. Similarly, the increased number of UCI bits also motivates the enhancements on UCI feedback on Physical Uplink Shared CHannel (PUSCH).
In LTE Rel-8, PUCCH format 1/1a/1b and PUCCH format 2/2a/2b are supported for Scheduling Request (SR), HARQ-ACK and periodic Channel State Information (CSI) reporting. The PUCCH resource is represented by a single scalar index, from which the phase rotation and the orthogonal cover sequence, only for PUCCH format 1/1a/1b, are derived. The use of a phase rotation of a cell-specific sequence together with orthogonal sequences provides orthogonality between different terminals in the same cell transmitting PUCCH on the same set of resource blocks.
In LTE Rel-10, PUCCH format 3 was introduced for CA and TDD, when there are multiple downlink transmissions, either on multiple carriers or multiple downlink subframes, but single uplink, either single carrier or single uplink subframe, for HARQ-ACK, SR and CSI feedback. Similarly, the PUCCH format 3 resource is also represented by a single scalar index from which the orthogonal sequence and the resource-block number can be derived. A length-5 orthogonal sequence is applied for PUCCH format 3 to support code multiplexing within one resource-block pair and a length-4 orthogonal sequence is applied for shorted PUCCH.
In LTE Rel-13, new PUCCH format design is ongoing to support a larger number of UCI bits.
In general, the exact relationship between the number of HARQ-ACK bits and the transmitted signals can be referred to as an encoding codebook of a CA PUCCH scheme. It is evident that the codebook needs to be synchronized between the UE and the evolved Node B (eNB, known collectively as BS hereinafter) for the HARQ-ACK feedback signal to be correctly processed and understood and processed on both sides. Basically, there are three different codebook adaptations:                Codebook adapted to the number of detected (Enhanced) PDCCHs (E)PDCCHs;        Codebook adapted to the number of activated Component Carriers (CCs);        Codebook adapted to the number of configured CCs.        
The first option is clearly problematic since the UE may miss detecting (E)PDCCHs from the BS, which immediately leads to divergence of codebooks assumed by the two sides. HARQ operations and status can thus be corrupted rather frequently.
The second option provides improved stability and reliability over the first option in the period between activation and de-activation of CCs. HARQ-ACK bit fields corresponding to CCs with no detected (E)PDCCHs are set to 0 (NAK) by the UE. The activation and de-activation of CC is performed via Medium Access Control (MAC) control elements. Due to HARQ feedback errors in the (de)-activation message, this signalling is not very reliable. In addition, CCs can also be autonomously and hence unilaterally de-activated by the UE based on UE-side timers. Therefore, basing the codebook adaptation on the CC activation state could be error prone.
The third option is a slow codebook adaptation and seems to be less efficient and have worse link performance than first two options at first glance. However, it has been shown that it has similar link performance as the first option 1 with smart BS decoding based on the fact that BS is aware of the scheduled and non-scheduled carriers. Therefore, it was agreed to be adopted in Rel-10.
Slow codebook adaptation is applied in LTE Rel-10, i.e. the HARQ-ACK codebook size for PUCCH format 3 is determined based on the number of configured CCs. In Rel-13, CA is enhanced to support up to 32 CCs, and slow codebook adaptation can still be applied to ensure the common understanding between the BS and UE. Smart decoding is also necessary so that the information of known bits can be utilized at the BS receiver. However, it should be noted that if there are many known bits compared to the unknown bits, the link performance of slow codebook performance will be degraded. On the other hand, among the 32 carriers, there are many unlicensed carriers which may not be accessible due to contention failure. It will not very efficient to determine the HARQ-ACK codebook size in a semi-static manner. Therefore, there is a motivation to use fast codebook adaptation.
It was proposed to use Downlink Assignment Index (DAI) signaling mechanism to indicate to the UE the number of scheduled carriers/subframes. The DAI in the DL assignment will be used to determine both the HARQ-ACK codebook size and the HARQ-ACK bit ordering. However, if the “last” grant(s) is missed by the UE, there will be misunderstanding between the BS and the UE regarding the HARQ-ACK codebook size. Consequently, the BS cannot correctly decode the PUCCH. One method to solve this problem is to do blind detection at the BS, e.g., the BS assumes that the UE missed certain (E)PDCCH assignments and attempts to decode the PUCCH for each assumed HARQ-ACK codebook size. The disadvantage of this method is that the BS needs to perform one or more additional decoding procedures. This is very complicated.